1. Field of the Invention
This invention relates to processes, continuous processes and related compositions for producing a more thermally stable flexible light pipe ("FLP") based on polymerized units of one or more acrylic esters, and the improved FLP product which the process produces.
2. Description of the Prior Art
An effective process for preparation of acrylic-based flexible light pipe is disclosed in two patents to Bigley et al., U.S. Pat. Nos. 5,406,641 and 5,485,541. In a preferred aspect of this process, a crosslinkable core mixture is present which comprises an uncrosslinked copolymer formed mainly from acrylic esters and monomers with functionally reactive alkoxysilane groups, along with a reactive additive to cure the uncrosslinked core polymer by crosslinking it, the reactive additive preferably being water and a silane condensation reaction catalyst, such as an organotin dicarboxylate. The core mixture is preferably polymerized by a bulk (non-solvent) process, more preferably by a continuous bulk process, the uncrosslinked copolymer preferably being devolatilized prior to co-extrusion with a cladding, preferably of a fluoropolymer, into a core/clad composite which is then separately cured to the final flexible light pipe.
The process based on a monomer such as ethyl acrylate taught by Bigley et al. yields a flexible light pipe or optical conduit which has high white light transmission, and acceptable flexibility and hardness for a variety of uses where light is to be conveyed from a remote source to a target and where the conduit needs to be flexible to follow a tortuous path, yet hard enough to retain its critical geometry.
The existing process further produces a FLP of adequate thermal and photothermal (joint exposure to heat and to visible light which may contain light of wavelengths known as the "near ultraviolet") stability even after exposures to long hours of light and ambient heat. The prior art polymer has adequate stability for exposure to higher temperatures, including those up to about 90.degree. C., for shorter use times.
However, there is a potential large market for light pipe which is thermally and photothermally stable at higher temperatures and longer exposure times, such as in automotive uses where the light is conducted near the engine compartment, and temperatures of 150.degree. C. or higher may be reached. Other potential uses where high temperatures may be encountered may be when the light source is not adequately shielded from the connection with the FLP, or where the light source is of extremely high intensity. Bigley et al. teach in general the use of stabilizers as part of the core component, but do not specifically teach or suggest an acceptable answer to this important stabilization problem.
We have discovered an improved process by which to prepare a crosslinkable acrylic core for a FLP which, after curing to crosslink, exhibits surprisingly improved stability to thermal aging while retaining its other desirable properties of good initial clarity, absence of initial color, good flexibility, adequate or somewhat improved photothermal stabilization, and adequate hardness to prevent physical distortion. An improved product, especially toward thermal aging in the absence of light being passed through the core, can be prepared by carefully controlling the temperature of the process, preferably shortening somewhat the residence time in the reactor, and controlling the nature of the initiator, so as to decrease the number of terminal vinyl groups in the polymer.